1. Field of the Invention
This invention relates to a polishing method for planarizing angular substrates, and particularly to a method for polishing substrates of quadrangular shape such as photomask substrates, liquid crystal substrates and disk substrates. The invention relates additionally to photomask blanks and photomasks obtained using substrates planarized by the inventive polishing process.
2. Prior Art
As the level of DRAM integration has continued to rise, so has the demand for ever smaller microcircuit geometries. This has led in turn to the use of exposure light of increasingly shorter wavelength. Shorter wavelength light increases resolution, but reduces focal depth. Hence, improving the focal depth has become an important concern. Because one major factor affecting focal depth is the flatness of the photomask substrate used in the lithographic process, a higher degree of flatness is desired in the photomask substrates from which photomasks are made. For instance, a flatness of at least 0.5 xcexcm, and especially 0.3 xcexcm or better, is desired in photomasks measuring 152xc3x97152 mm.
Photomask substrates are usually made of synthetic quartz glass. The production of synthetic quartz glass substrate is briefly described. First, a glass ingot is formed by flame hydrolysis using gases generated from a starting material such as silicon tetrachloride and an oxyhydrogen flame. The ingot is melted under heating and molded into an angular shape. The angular ingot is cut into slices, which are then polished several times to increasing levels of precision, ultimately giving photomask substrates.
Such polishing is generally performed by polishing multiple slices at once on a double side polishing machine. Double-sided polishing is widely used in large part because, in addition to being beneficial for mass production, it also happens to be effective against scratching. Exposure light is passed through the photomask, so both sides of the photomask substrate from which the photomask is made must be free of scratches and other defects.
Double-sided polishing is described while referring to attached FIG. 1. Two polishing turntables 3 and 3xe2x80x2, respectively composed of a lower platen 1a and an upper platen 1b to each of which is bonded a polishing pad 2, press from both sides against substrates to be polished 11 which have been inserted by a carrier (not shown). Polishing is carried out by independently rotating the turntables 3 and 3xe2x80x2. This arrangement allows both sides of the substrates 11 to be polished at the same time, which is effective for preventing scratches. FIG. 2 is a schematic view showing only one turntable 3 and one substrate to be polished 11. The substrate 11 rotates at the center, and so its surface is polished to a concentric shape.
When the substrate 11 is pressed against the polishing pad 2 during polishing, as shown in FIG. 3, the substrate 11 sinks into the pad 2 under the pressing force. At the start of such sinking, the polishing pad 2 exerts a large elastic force upon the substrate 11, increasing friction by the polishing pad 2 at places 11a (shaded areas) which correspond to the outside of the inscribed circle on the substrate 11 shown in FIG. 2. Moreover, because the substrate 11 being polished is an angular substrate, rotation of the substrate 11 subjects the shaded areas 11a to the successive application and release of pressing forces, vastly increasing the opportunities for the substrate 11 to incur the elastic forces of the polishing pad 2 compared to a circular substrate. The result is excessive abrasion of the substrate, rapid degradation at corresponding places on the polishing pad 2, and the emergence of unevenness in the elasticity and other characteristics of the polishing pad 2. Such unevenness in the properties of the polishing pad 2 prevents the substrate 11 from being polished at a uniform rate, as a result of which the surface of the substrate 11 is polished non-concentrically. In FIG. 3, the lengths of the arrows indicate the relative magnitude of the forces of restitution acting under the elasticity of the polishing pad 2.
Also, when a plurality of substrates are polished at the same time, the existence of variations in the thickness of the substrates prevents a uniform load from being applied to the substrates. Hence, the rate at which any one substrate is polished is not uniform, which tends to result in non-concentric polishing of the substrate surface.
In angular substrates, the polishing speed in peripheral areas thus tends to be too rapid, making it difficult to stably manufacture flat substrates. This has significantly lowered production yields.
Moreover, once the surface of a substrate has been non-concentrically polished, subsequent repair is difficult and it is essentially impossible to obtain a substrate of high planarity. This too has greatly lowered yields.
A need has thus been felt for a way to polish angular substrates which enables the stable production of angular substrates of high planarity by preventing the excessive removal of material in peripheral areas and controlling the polishing rate in the plane of the substrate.
Also, to obtain substrates which have been effectively planarized from the non-concentrically polished state that often arises in the prior art such as with the use of a double side polishing machine, there has been a desire for a way to produce planarized substrates, regardless of the substrate shape prior to polishing, by selectively polishing selected areas of the substrate.
Another important consideration is the importance at all times of keeping scratches and other defects from arising in the substrate during polishing.
It is therefore one object of the invention to provide a polishing process which is capable of finishing the surface of an angular substrate to a high degree of flatness and does not cause scratches and other defects to form, and particularly a polishing process which minimizes excessive material removal at the corners of the substrate.
It has been found that the problems described above can be overcome by polishing the substrate on a polishing machine while pressing the substrate and also similarly pressing a guide ring against the polishing pad.
Accordingly, in a first aspect, the invention provides a method for polishing an angular substrate having a surface to be polished, which method involves the steps of holding the angular substrate with a substrate holding head having a guide ring such that the substrate fits in the guide ring; pressing the substrate surface to be polished, and also one surface of the guide ring, against a polishing pad; and independently rotating the polishing pad and the substrate holding head together with the substrate it holds, while pressing the polishing pad-contacting surface of the guide ring against the polishing pad, to polish the substrate surface.
The substrate holding head typically has a holding side for holding a back surface of the substrate, and include an elastomer disposed on the holding side such as to contact primarily only peripheral areas of the back surface of the substrate.
The guide ring is preferably of a size which includes therein a circle whose diameter is equal to the diagonal of the substrate.
The substrate polishing method may employ one mechanism to apply a pressing force to the substrate and a different and independent mechanism to apply a pressing force to the guide ring.
The guide ring may be in one piece or divided into segments. If divided into segments, the divided guide ring is generally configured to allow a pressing force to be applied independently to each segment.
The ratio A/B between the pressing force A applied to the guide ring and the pressing force B applied to the substrate preferably satisfies the condition: 0 less than A/Bxe2x89xa65.
A surface portion of the guide ring in contact with the polishing pad is preferably made of a material that is a major constituent of the substrate. Both the substrate to be polished and the guide ring surface portion may be made of synthetic quartz glass.
The substrate in the method of the invention may be a photomask substrate.
In another aspect, the invention provides a photomask substrate obtained by the above-described substrate polishing method of the invention.
In a further aspect, the invention provides a photomask blank produced from the foregoing photomask substrate.
In a still further aspect, the invention provides a photomask produced from the foregoing photomask blank.